1. Field of the Invention
The present invention relates to audio reproduction systems and particularly to audio reproduction systems suitable in practice for reproduction rooms of variable size, such as cinemas, wherein the audio reproduction systems are based on the wave-field synthesis.
2. Description of the Related Art
There is an increasing demand for new technologies and innovative products in the field of consumer electronics. Thereby, it is an important prerequisite for the success of new multimedia systems that they offer optimum functionalities and capabilities, respectively. This is achieved by the usage of digital technologies and particularly computer technique. Examples therefore are applications providing an improved realistic audio visual impression. Conventional audio systems have a significant weak point in the quality of the spatial sound reproduction of natural but also virtual environments.
Methods for multi channel loudspeaker reproduction of audio signals have been known and standardized for many years. All common techniques have the disadvantages that both the site of the loudspeakers and the position of the listener are already impressed onto the transmission format. With a wrong arrangement of the loudspeakers with regard to the listener, the audio quality suffers significantly. An optimum sound is only possible in a small area of the reproduction room, the so-called sweet spot.
A better natural spatial impression as well as stronger enclosure in the audio reproduction can be obtained with the help of a new technology. The basics of this technology, the so called wave-field synthesis (WFS) have been researched at the TU Delft and have been presented for the first time in the late 80ies (Berkhout, A. J.; de Vries, D.; Vogel, P.: Acoustic control by Wave-field Synthesis. JASA 93, 1993).
Due to the huge requirements of this method with regard to computing effort and transmission rates, the wave-field synthesis has hardly been applied in practice so far. Only the progresses in the field of microprocessor technique and audio encoding allow the usage of this technology today in specific applications. First products in the professional field are expected next year. In a few years, the first wave-field synthesis applications will come on the market for the consumer field.
The basic idea of WFS is based on the application of the Huygens principle of the wave theory:                Every point captured by a wave is the starting point of an elementary wave which propagates in a spherical or circular way.        
Applied to acoustics, any form of an incoming wave front can be reproduced by a large number of loudspeakers arranged next to one another (a so-called loudspeaker array). In the simplest case, a single point source to be reproduced and a linear arrangement of the loudspeakers, the audio signals of every loudspeaker with a time delay and amplitude scaling have to be fed such that the emitted sound fields of the individual loudspeakers overlay properly. With several sound sources, the contribution to every loudspeaker is calculated separately for every source and the resulting signals are added. If the sources to be reproduced are in a room with reflecting walls, reflections also have to be reproduced via the loudspeaker array as additional sources. Thus, the effort in calculating depends strongly on the number of sound sources, the reflection characteristics of the recording room and the number of loudspeakers.
The particular advantage of this technique is that a natural spatial sound impression is possible across a large range of the reproduction room. In contrary to the known techniques, direction and distance from the sound sources are reproduced very exactly. To a limited degree, virtual sound sources can even be positioned between the real loudspeaker array and the listener.
Although the wave-field synthesis functions well for surroundings whose conditions are known, irregularities occur when the conditions change and when the wave-field synthesis is performed based on a surroundings condition, which does not correspond to the actual condition of the surroundings.
A surrounding condition can also be described by the impulse response of the surroundings.
This will be explained in more detail with regard to the following example. It is assumed that a loudspeaker emits a sound source signal against a wall whose reflection is undesirable. For this simple example, the room compensation by using the wave-field synthesis would be that first a reflection of this wall is determined in order to determine when a sound signal that has been reflected by the wall reaches the loudspeaker again and what amplitude this reflected sound signal has. When the reflection from this wall is undesirable, the wave-field synthesis offers the possibility to eliminate the reflection from this wall, by impressing a signal opposite in phase to the reflection signal into the loudspeaker with a corresponding amplitude, additionally to the original audio signal, so that the forward compensation wave eliminate the reflection wave, such that the reflection from this wall is eliminated in the surroundings that are considered. This can take place by first calculating the impulse response of the surroundings and determining the condition and position of the wall based on the impulse response of these surroundings, wherein the wall is interpreted as mirror source, which means as sound source reflecting an incident sound.
If, at first, the impulse response of these surroundings is measured and then the compensation signal is calculated, which is to be impressed to the loudspeaker overlaying the audio signal, an elimination of the reflection from this wall will take place, such that the listener in these surroundings will have the impression that this wall does not exist at all with regards to sound.
However, it is fundamental for an optimum compensation of the reflective wave that the impulse response of the room is determined exactly, so that no over- or undercompensation occurs.
Thus, the wave-field synthesis enables a correct mapping of virtual sound sources across a large reproduction range. At the same time, it offers new technical and creative potential to the recording engineer and sound engineer for the design of complex sound scenes. The wave-field synthesis (WFS or also sound-field synthesis), as it has been developed at the end of the 80ies at the TU Delft, represents a holographic approach of sound reproduction. The Kirchhoff Helmholtz integral serves as basis for this. It indicates that arbitrary sound fields within a closed volume can be generated via distribution of monopole and dipole sound sources (loudspeaker arrays) on the surface of this volume. Details about that can be found in M. M. Boone, E. N. G. Verheijen, P. F. v. Tol, “Spatial Sound-Field Reproduction by Wave-Field Synthesis”, Delft University of Technology Laboratory of Seismics and Acoustics, Journal of J. Audio Eng. Soc., Vol. 43, No. 12, December 1995 and Diemer de Vries, “Sound Reinforcement by Wavefield Synthesis: Adaption of the Synthesis Operator to the Loudspeaker Directivity Characteristics”, Delft University of Technology Laboratory of Seismics and Acoustics, Journal of J. Audio Eng. Soc., Vol. 44, No. 12, December 1996.
In wave-field synthesis, a synthesis signal is calculated for every loudspeaker of the loudspeaker array from an audio signal emitted by a virtual source at a virtual position, wherein the synthesis signals are formed such with regard to amplitude and phase that a wave resulting from the superposition of the sound waves output by the individual loudspeakers present in the loudspeaker array, corresponds to the wave that would originate from the virtual source at the virtual position, when this virtual source at the virtual position would be a real source with a real position.
Typically, several virtual sources are present at different virtual positions. The calculation of the synthesis signals is performed for every virtual source at every virtual position, so that typically one virtual source results in synthesis signals for several loudspeakers. Thus, seen from a loudspeaker, this loudspeaker receives several synthesis signals originating from different virtual sources. A superposition of these sources, which is possible due to the linear superposition principle, results then in the reproduction signal actually emitted by the loudspeaker.
The possibilities of wave-field synthesis can be utilized the better the larger the loudspeaker arrays are, i.e. the more individual loudspeakers are provided. However, this increases also the computing power that a wave-field synthesis unit has to perform since, typically, channel information has to be considered as well. This means that from every virtual source to every loudspeaker, basically, an individual transmission channel is present, and that, basically, the case can exist that every virtual source leads to a synthesis signal for every loudspeaker and that every loudspeaker obtains a number of synthesis signals, which is equal to the number of virtual sources, respectively.
If the possibilities of wave-field synthesis are to be exhausted in that the virtual sources can also be moveable, particularly in cinema applications, it can be realized that significant computing efforts have to be mastered due to the calculation of synthesis signals, the calculation of the channel information and the generation of the reproduction signals by combining the channel information and the synthesis signals.
Above that, it should be noted here that the quality of audio reproduction increases with the number of provided loudspeakers. This means that the audio reproduction quality becomes the better and the more realistic the more loudspeakers are present in the loudspeaker array(s).
In the above scenario, the fully rendered and analog-digital converted reproduction signals for the individual loudspeakers can, for example, be transmitted via two-wire lines from the wave-field synthesis central unit to the individual loudspeakers. This would have the advantage that it is almost guaranteed that all loudspeakers operate synchronously, so that no further measures would be required for synchronization purposes. On the other hand, the wave-field synthesis central unit could always only be produced for a specific reproduction room and for a reproduction with a fixed number of loudspeakers, respectively. This means that an individual wave-field synthesis central unit would have to be produced for every reproduction room, which has to provide a significant amount of computing power, since the calculation of the audio reproduction signals, particularly with regard to many loudspeakers and many virtual sources, respectively, has to be performed at least partially in parallel and in real time.
Particularly with regard to audio reproduction systems intended for cinemas, there is the problem that the reproduction rooms in cinemas vary significantly with regard to their size. Cinemas sometimes have a very large cinema screen and/or at the same time several small cinema screens for films having not such a high number of viewers as films to be played on large cinema screens. But different cinemas have differently sized reproduction rooms, which can vary possibly up to a factor 100, particularly when an audio reproduction is considered not only for cinemas but also, for example, for concert halls.
In order to equip such different audio reproduction rooms with an audio reproduction system based on wave-field synthesis, e.g. an individual wave-field synthesis central unit would have to be built for every reproduction room, which is not acceptable with regard to the price due to the individual production.
On the other hand, a maximally equipped wave-field synthesis central unit could be constructed, which is controllable with regard to the connectable loudspeakers, which means with regard to the number of analog signal outputs, but internally comprises computing processors, which is designed for the maximum number of analog outputs, which means connectable loudspeakers.
Such a system would lead to the fact that audio reproduction systems for smaller reproduction rooms have almost the same price as audio reproduction systems for very large reproduction rooms, which will probably not be acceptable for the operators of small reproduction rooms. Particularly medium to small reproduction rooms are interesting for providers of audio reproduction systems, wherein the “smallest” reproduction rooms should also be mentioned, which are, for example, private living rooms or smaller restaurants and bars.
Thus, the above-described possibilities are disadvantageous and that a radical market acceptance can not immediately be expected.